I don't get once a band has enough material to play live why they would still do covers. Maybe a musician can fill me in on that one

Honestly, I think part of it is just the fun of it. I have a lot more fun learning and playing cover songs than I do playing my own stuff most of the time. When you're in a band, your own music definitely becomes "work" to an extent - between writing, rehearsing, recording, and playing it live. I can TOTALLY understand a band wanting to just have some fun and throw in a cover song or two. I just think it adds a certain unexpected element to it.

When my band releases our full-length album next year, I'm planning a headlining CD release show where we play the whole album front-to-back (about 55 minutes) and then end the night with a couple of covers, including a few "classics" (Deep Purple's Burn, Sabbath's Heaven and Hell). It's just gonna be a really fun way to end the night for everyone.